This invention is in the field of gate valves for water and waste water service. More particularly, the invention is in the field of resilient-seated gate valves, e.g., those covered by ANSI-AWWA specification C-509.
Resilient seated gate valves have metal gates either coated with rubber to form a resilient seat, i.e., seal, or they have a gate with a rubber seat either mechanically attached or bonded to the gate. The seat portion seals against the body of the valve after being closed by the well known means of a threaded stem and nut. The gate is guided as it descends into the valve body by grooves in the gate and mating protrusions on the body or the reverse thereof, i.e., grooves in the body and protrusions on the gate. Since such valves are almost exclusively used for water or waste water service, they are usually made of iron and the body portion is usually completely coated inside with a corrosion resistant coating. The preferred inside coating for these valve bodies is an epoxy suitable for contact with drinking water. While an inside epoxy coating is superior to almost all others, even the epoxy can be worn off the guide surfaces after relatively few operations of the valve in service. The guide surfaces with their corrosion resistant coatings thus removed are subject to corroding. This corroding might not be a serious hazard to the valve except that it occurs on guide surfaces which must slide on either rubber coated or uncoated iron surfaces. A buildup of corrosion on these surfaces can cause excessive torque requirements to operate the valve and, in extreme cases, cause failure of the operating mechanism. The importance of the invention herein disclosed is magnified when it is considered that valves in a water system may remain closed for long periods of time before operation is required. Corrosion of the guide surfaces and the thousands of pounds of force against one side of the gate makes a redoubtable task of manually opening such a valve. A valve equipped with the present invention will, however, open with ease since it will have its corrosion resistant coating system intact even though it has been operated in service many times.